Dynamically Sized Table View Header or Footer Using Auto Layout

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    // Dynamic sizing for the header view
    if let headerView = tableView.tableHeaderView {
        let height = headerView.systemLayoutSizeFittingSize(UILayoutFittingCompressedSize).height
        var headerFrame = headerView.frame

        // If we don't have this check, viewDidLayoutSubviews() will get
        // repeatedly, causing the app to hang.

        if height != headerFrame.size.height {
            headerFrame.size.height = height
            headerView.frame = headerFrame
            tableView.tableHeaderView = headerView
        }
    }
}

http://collindonnell.com/2015/09/29/dynamically-sized-table-view-header-or-footer-using-auto-layout/